Wednesday 3 March 2010 19.05 EST
First published on Wednesday 3 March 2010 19.05 EST

At conferences this week, the Liberal Democrats and the Conservatives are talking up a shared big idea, the "pupil premium" – a slice of extra cash that every poor child carries with them through the school gate. It is a mark of Labour's success in nudging the debate leftwards that this has become the new electoral battleground; only a few years ago the Tories were arguing about whether proposed educational vouchers could be put towards public school fees. But it bears testament, too, to the Brown government's abject failure to give a comprehensible account of itself that the opposition should find such easy pickings.

For the first and least-familiar finding of a new Institute for Fiscal Studies analysis of the premium proposal is that there are already big anti-poverty payments in the school system. Through a mish-mash of Whitehall grants and local funding formulas, poor secondary pupils already benefit from annual funding that exceeds the average by around £3,400 annually. This implicit premium has long recognised the additional barriers to learning faced by children with cash-strapped home lives, but it has recently grown extremely sharply, and as it has done so the stubborn GCSE attainment gap between rich and poor children has finally narrowed a little. It is remarkable that it should fall to the IFS rather than the government to spell out these heartening facts. After all, as the opposition rightly argue, the premium must be clear and simple if it is to offset the temptation for good institutions to encourage a more affluent intake. While the government remains silent about its own pupil premium, headteachers often remain ignorant of it, so it is not doing the good that it might.

The united opposition front fractures on the question of money. The cerebral Liberal Democrat spokesman, David Laws, is explicit. By earmarking savings from controversial tax-credit cuts, he proposes to inject £2,500 more on behalf of every poor pupil. The Conservatives, by contrast, have nothing to say about how much money they would find, or from where. They fail even to say the existing schools budget is safe, which comes close to admitting the axe will fall.

David Cameron must now explain how he will avoid the obvious trap pointed out by the IFS – that with a fixed overall budget, premium payments in some classrooms must translate into cuts in others. In any hung parliament, the shared rhetoric of pupil premiums will provide an obvious starting point for the Liberal Democrats and the Conservative to talk. On the basis of what we have heard from this week, however, the conversation might falter the moment that it turns to hard cash.